Sample data for use with ltmleMSM. Data: n=1000: male age CD4_1 A1 Y1 CD4_2
A2 Y2 CD4_3 A3 Y3 A1..A3 are treatment nodes, Y1..Y3 are death, CD4_1..CD4_3
are time varying covariates. We are interested in static regimes where a
patient switches at some time. In summary.measures, switch.time is first
time where At is 1 (4 if never switch), time is the horizon.